Aerodynamic density meter using photosensitive schlieren optical system



W. R. GRABO DENSITY METER 3,101,414 ITIVE WSKY Aug. 20, 1963 AERODYNAMIC USING PHoTosENs SCHLIEREN OPTICAL SYSTEM '7 Sheets-Sheet 1 Filed Sept. 28, 1961 Aug. 20, 1963 w. R. GRABowsKY 3,101,414

AERoDyNAMTc DENSITY METER USING PHoToSENsTTIvE SCHLIEREN OPTICAL SYSTEM 7 Sheets-Sheet 2 Filed Sept. 28, 1961 INVENToR. Maw/5 i?, /moq/ky Aug. 20, 1963 w. R. GRABowsKY 3,101,414

AERODYNAMIC DENSITY METER USING PHoTosENsITIvE SCHLIEREN OPTICAL SYSTEM Filed Sept. 28, 1961 7 Sheets-Sheet 3 mr/ENToR. Wsw/5 E. f5/awww l l I I I I I I I I I I I I I I I INVENToR. MM z/f /K Jimena/Ky Zig/4 inv/@yay Aug. 20, 1963 AERODYNAMIC Filed sept. 28, 1961 n mHTHTTMTMTMT#H TMTL@ 3,101,414 TosENsITIvE Aug. 20, 1963 w. R. GRABowsKY AERODYNAMIC DENSITY METER USING PHo scHLIEREN OPTICAL SYSTEM Filed sept. 2s, 1961 7 Sheets-Sheet 5 W. R. G RABOWSKY DENSITY Aug. 20, 1963 3,101,414 AERODYNAMIC METER USING PHoTosENsITIVE SCHLIEREN OPTICAL SYSTEM 7 Sheets-Sheet 6 Filed Sept. 28, 1961 INVENTOR.

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AERODYNAMIC DENSITY METER USING PHoTosENSITIvE SCHLIEREN OPTICAL SYSTEM 7 Sheets-Sheet '7 Filed Sept. 28. 1961 INVENIoR. Mau f?. @w50/mw Pw 5M United States Patent O AERODYNAMIC DENSITY METER USlNG PHOTO- SENSI'Il/'E SCHLIEREN OPTICAL SYSTEM Wallis R. Grahowsky, an Pedro, Calif., assigner to the United States of America as represented` by the Secretary of the Navyy Filed Sept. 28, 1961, Sel'. No. 141,554

7 Claims. (Cl. Z50- 218) This invent-ion relates to an aerodynamic density meter and especially to a direct-reading instrument of high sensitivity which is particularly useful for measuring 'density changes 'associated with low-speed aerodynamic ilows.

Heretofore, quantitative density measurements associated with aerodynamic flows have been made by means of sh-adowgraphs, Schlieren -or interferometers. These devices all require a photographic recording of results 'which makes it impossible to obtain immediate precise, quantitative information while a -study is `in progress. The shadowgraph is inherently a low-sensitivity device and is thus used exclusively for flow field visualization where density gradients are large, eg., shock phenomena in high pressure iiows. Ihe schliefen system is in theory a highly sensitive device, but the high sensitivity is usually lost in photographic recording land interpretation. The interferometer is less sensitive than the Schlieren, is very diicult to set up, and is quite expensive.

The need for extreme sensitivity becomes even more acute when `density changes in low-speed aerodynamic ows are to be determined because :density changes will be extremely small. This application describes -an instrument capable of interpreting this density change as a proportional voltage diierence. Such `small voltage differences are easily measured. Y

Consider a signal voltage, V-l-AV. f V=5 volts and AV: .000005 volts, measurement of V-l-Al/:SQOOGOS volts is impossible '(without a null system) since this requires a meter with at least 5 major divisions of l'volt, each of these being subdivided into 1010,00() subintervals. However, a way to obviate this difficulty is to use the method of difference measurement. Here a reference voltage, V=5.0\O000' volts is subtracted from the signal voltage, V+AV=5.000005 volts, to obtain the ditference, AV=10000O volts. This permits the use of a meter which reads .00001 volts `full scale and reading AV becomes easy.

The objects and advantages of the present invention are accomplished by means of a split-beam ySchlieren syste-m in combination with an electronics system which converts the two light beams into corresponding pulsating DC., signal voltages, transforms the pulsating D.C. signals into AC. signals, trectilies these A.C. signals and eliminates most of the noise associated lwith the signal voltages, changes the rectified A.C. signals into corresponding steady D.C. voltages 'and subtracts the D C. voltages to ob-tain .their difference.

The object of the invention is -to provide a Schlierentype instrument for studying aerodynamic ilow,fsaid instrument having high'sensitivi-ty, being capable of measuring the density changes of steady and unsteady flows, having quick readout for steady flow applications, -being applicable to density measurements of ilow speeds. ranging from slow to hypersonic, and being fairly inexpensive to construct.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. l is a diagrammatic illustration, partly in block form, of an embodiment of the invention;

FIG. 2 is la diagrammatic representation of the chop- 3,10 l ,414 Patented Aug. 20, 1 963 per wheel and the chopper-wheel cover plate, the chopper wheel being behind the cover plate, and only suicien-t slots being shown to illustrate the geometrical rellationships between Ithe slotsfand the windows;

FIG. 3 is a representation of the photomultiplier output wave form;

FIG. 4 is a representation illustrating the relationship between the photom'ultipl-ier output signal and the switching signal;

FIG. 5 is a representation of the output signal of the synchronous detector; and 1 FIGS. 6A through 46D are schematic circuit diagrams of the electronic portions of Ithe embodiment of the invention shown in FIG. l.

The basic concept of the invention is illustrated in the schematic diagram, FIG. l. Here a standard schliefen system 12, comprising a light source 14 providing Ia one millimeter square image, a pair of"f2.5 lenses 16 and 18, a knife edge 20 and a collimating lens 22, projects a collimated beam of light lthrough windows in a stationary chopper-wheel kcover plate 25 upon a rot-ating chopper wheel Z4.

The chopper wheel 24 indicated n FIG. 2 has twenty slots 23 spaced 18 degreesapart, the distance between the slot centerlines at center radius being four slot widths (4d). The chopper-wheel cover 'plateV 25 has two rectangular openings or windows 26 and 2S, the centerlines at the Windows being separated by 9 degrees. The width (d) of each of the rectangular windows 26 and 28 is the same as the width (d) of a slot Z3. These windows effectively divide the light beam into two halves, A and B,

.- which impinge upon the photomultiplier 30, and are alternately interrupted by the action yof the rotating chopper wheel.. The photomultiplier output signal is ideally indicated in FIG. 3 and may be seen to be a pulsating D.C. signal with a repetition rate (or, 'as it will be termed hereinafter 5frequency) of about 1200 c.p.s.

The chopper wheel cover plate 25 also has a third opening 27 which is sinusoidally shaped onone of its circumferential sides and aligned with the chopper wheel slots 26 and 28. Light from light source 32 passes through this opening `and falls upon a photocell 34 which provides =a sine waveoutput signal. The, size and spacing of this opening 27 produces |a sine wave of frequency equal to the frequency of the output pulses of either light beam, A or B, or halfthe frequency of .the output signal of the photomultiplier 3l). The sine wave signal is fed through an amplier Iand cathode follower stage 36 and a phase shifter 38, so that the zer-o voltage points of the sine wave and the photomultiplier output signal can be made to coincide. The phase relationship of the two waves is shown in FIG. 4,' where the phase-shifted sine wave is indicated Iby numeral 39` land the photomultiplier output signal by numeral `411.

The output signal of the phlotomultiplier 30 and the phase-shifted sine wave signal are applied yto a synchronous detector 40. The sine wave signal derived from the photocell 34 will hereinafter be referred to as the switching signal. The synchronous detector 40 is an electronic switch, the switching 'action of which is effected by the switchingl signal. The synchronous detector 49 changes the pulsating D C. signal from the photomultiplier 30 to an A.C. signal 43,- which has a z'er'o -D.C. level if the signals from beam A and beam B larey equal. If the medium through which one beam travels is different in density gradient from the medium through :Which the I other -beam travels, the output from one -beam will differ in amplitude from the output lof the other beam. A density change in either Vbeam causes an average 'current to iiow from the detector. The normal A C. signal (i.e., the A.C. signal which results when the outputs of the two beams are equal) that appears at the output of the syn- 120 33 210 5 130 7.5M 220 100 132 10M 224 v 4.5 134 mmf 500 22s v 4.5 136 500 230 614 13s 5M 231 V 180 140 mmf 160 232 614 144 33 234 0.1 146 mmf 500 23s 20 14s 0.6 236 0.01 160 20 23s 1 11a 10 26s 614 120 0.01 270 v 180 166 6116 216 1/2-61-16 sa 61-16 21s 300 111e 6116 2a 300 192 6116 282 mmf 200 Obviously many modifications and variations of Ilthe present invention are possible in the light :of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may' be practiced otherwise than as specifically described.

l. An instrument for the measurement of density diffeernces in a medium exposed-to an aerodynamic flow comprising, in vcombination: schlieren :optical means for projecting a colli-mated beam of light through said medium; means for dividing said light beam linto two parts and for interrupting each part alternately; means for producing an electrical output signal proportional to light intensity, said light beam being directed at said signalproducing means to provide an electrical output comprising 1a pulsating D.C. signal every second pulse of which is proportional to the intensity of the `same part of said light beam, -alternate pulses being derived from different parts of said light beam; means for obtaining an A.C. signal having a characteristic proportional to la characteristic of said pulsating D.C. signal; narrow-bandwidth means for passing said A C. signal and eliminating frequencies other than a narrow band including said ALC. signal, thereby eliminating the spectrum of noise signals outside said narrow band; means forseparating said A.C. signal linto individual components, each corresponding to the output associated with a different one lof the parts of said light beam, said components comprising steady D.C. signals; and means -f-or obtaining and indicating the difference between said steady D.C. signals.

2. An instrument for the measurement of density differences in a me-dium exposed to yan aerodynamic flow comprising, in combination: Schlieren optical means for projecting a collimated beam of light through said medium; means for dividing `said light beam into two halves and for interrupting each half alternately; means for producing an electrical output signal proportional to light intensity, said light lbeam being directed at said signal-producing means to provide lan electrical output comprising a pulsating DC. signal every second pulse of which is proportional to the intensity of the same Ihalf of said light beam, alternate pulses :being derived from different halves of said light beam; means for-obtaining an A.C. signal having a frequency proportional to that of said pulsating D.C. signal; narrow bandwidth means for passing said A.C. signal and eliminating frequencies other than a narrow band including said A C. signal, thereby eliminating the spectrum of noise signals outside said narco-w band; means for separating said A.C. signal into individual components, each corresponding to the output associated with a diierent one of the halves of lsaid light beam, said components comprising steady D.C. signals; and means for obtaining and indicating the difference between said steady D.C. signals.

3. An instrument for the measurement o-f density differences in a medium exposed to an aerodynamic liow comprising, in combination: a Schlieren optical system including a light beam traversing said medium; means for dividing said light beam into two parts and for interrupting each part alternately; vtirst means for producing an electnical output signal proportional to light intensity, said light beam being directed at said first means to provide an electrical output comprising a pulsating DLC. signal every second pulse' of which is proportional to the intensity of the same ypart of said light beam; alternate pulses being derived vfrom different parts of said light beam; second means for obtaining an electrical output signal, said second signal being an A.C. signal having a frequency equal to the repetition rate of the 'electrical pulses derived from either of said parts of said light beam; means for phasing oneof said electrical output signals so that selected points of the waves are caused to coincide; means for transforming said two phased electrical output signals into a single A.C. signal having a characteristic proportional to a characteristic of said pulsating D C.v signal from said rst means; means, to whch'said A.C. signal is' applied, rfor passing only a narrow band of frequencies which includesthat of said A.C. signal, noise outside of said band of frequencies thereby beingeliminated; means for producing a pair of rectified signals from said narrow-band A.C. signal,"'the rst rectified signal corresponding to the output signal from one part of said light beam'and the other rectified signal corresponding'to the output signal from the other part of said light beam; means for deriving a steady D.C. signal from each of said rectified A.C. signals; and means for obtaining and indicating the difference between said steady'D1C. signals.

4. An instrument for the measurement of density v-differences in a medium exposed to an aerodynamic iiow comprising, in combination: a Schlieren optical system including a light beam traversing salid medium; means for dividing said light beam into two yhalves and for interrupting each half-alternately; irst means for producing an electrical output signal proportional to light intensity, said light beam being directed at said first means 'to provide an electrical output comprising a pulsating D.C. signal every second pulse of which is proportional to the intensity of the same half ofs'aid light beam, alternate pulses being Y derived from different halves of said light beam; second means for producing an electrical output signal, said sec-A ond signal being an A.C.k signal having a frequency equal to the repetition rate of the electrical pulses derived from either of said halves of said light beam; means for phasing one of said electrical output signals to that selected points of waves are caused to coincide; means for transforming said two phased electrical output signals into a single A.C. signal having a characteristic proportional to a characteristie of said pulsating DC. signal from said rst means; means, to which said A.C. signal is applied, for passing only a narrow band of frequencies which includes that of said A.C. signal, noise outside of said band of frequencies thereby being eliminated; means lfor producing a pair of rectiiied signals from said narrow-band A.C. signal, the first rectified signal corresponding to the output signal from one half of said light beam and the other rectified signal corresponding to the output signal from the other half of said light beam; means for deriving a steady DC. signal from each of said rectiiied A.C. signals; and means for obtaining and indicating the difference between said steady D C. signals.

5. An instrument for the measurement of density differences in a medium exposed to an aerodynamic tlow comprising, in combination: a Schlieren optical system including a light beam traversing said medium; means for dividing said light beam into two halves and `for interrupting each half alternately; rst means for producing an electrical output signal proportional to light intensity, said light beam being directed at said rst means to provide an electrical output comprising a pulsating D.C. signal every second pulse of which is proportional to the intensity of the same half of said light beam, alternate pulses' being derived from different halves of said light beam; means for producing an electrical output signal, said second signal being an A C, signal having a frequency equal to the repetition rate oi the electrical pulses derived from either of said halves of said light beam; means for shifting the phase of said A.C. signal so that the zero voltage points of the AC. and pulsating DC. signals coincide; means for transforming said two phased electrical output signals into a single AC. signal having a characteristic proportional to a characteristic of said pulsating DC. signal from said first means; means, to which said A.C. signal is applied, for passing only a narrow band ot frequencies which includes that of said A,C. signal, noise outside of said band of frequencies thereby being eliminated; means for producing a pair of rectied signals from said narrow-band AC. signal, the rst rectified signal corresponding to the output signal from one half of said llight beam 'and the other rectilied signal corresponding to the output signal from the other half of, said light beam; means for deriving a steady D C. signal from each of said rectified A.C. signals; and means for obtaining and indicating the difference between said steady DC. signals.

6. An instrument -for fthe measurement of density dictference tin a medium exposed to an aerodynamic flow comprising, in combination: a Schlieren optical system including a light beam travers-ing said medium; means for dividing said light beam into two halves and for interrupting each half alternately; lirst means for producing an electrical output signal proportional to light intensity, said light beam being directed at said rst means to provide an electrical output comprising a puls-ating D C. signal every second pulse of which is proportional to the intensity of the same half of said light beam, alternate pulses being derived from different halves of said light beam; means for producing an electrical output signal, said second signal being 'an A.C. signal having -a frequency equal to the repetition rate of the electrical pulses derived `from either of said halves of said light beam; means for shifting the phase of said A C. signal so that the zero voltage points of the A.C. and pulsating D.C. signals coincide; means for transforming said two phased electrical output signals into a single A C. signal having its positive crest amplitude proportional to the amplitude of the signals from one half oisaid light beam and its negative crest amplitude proportional to the arnplitude of the signals from the other half of said light cam; means, to which said AC. signal is applied, for passing only a narrow band of frequencies which includes that of said A C. signal, noise outside of said band yof frequencies thereby being eliminated; means for producing a pair of rectiiied signals from said narrow-band A.C. signaL'the first rectified signal corresponding to the output signal from one half of said light beam and the :other rectiiied signal corresponding to the output signal from the other half of said light beam; means tor deriving a steady D C. signal from each of `said rectified A C. signails; land means for obtaining and indicating the difference between said steady D.C. signals.

7. An instrument for the measurement of density dittfe-rences in a medium exposed to an aerodynamic flow comprising, in combination: a Schlieren optical system including :a light beam traversing said medium; a rotatable, opaque chopper wheel having a set of equally spaced, substantially rectangular, peripheral slots, the width of each slot at its center being equal to a dimension (d) and `the distance between the slots `at their centers being equal to Mal); a stationary opaque plate concentric with and shielding said chopper wheel from said light beam, ysaid plate having a pair of substantially rectangular windows located approximately in `axial correspondence with said chopper slots, the width of said Windows being equal to the width (d) of said slots and the adjacent edges of said windows being spaced from each other by an amount (d said plate also having a third substantially rectangular window with Ione substantially sinusoidally shaped side extending in the same direction as the circumjerence of said chopper Wheel, the long dimension of said third window extending in the circumferential `dimension of said chopper wlheel from the center of one said slot -to the center of an adjacent slot; a photomultiplier having a light-responsive element, said chopper wheel and said opaque plate being located between said light beam and said light-responsive element, .said light beam being lowed to impinge upon said chopper-Wheel slots by said first and second windows; photocell means and :a light source, said chopper Wheel being located therebetween, light yfrom said light source being allowed to impinge upon lthe photocell by said sinusoidal Window; phase-shifting means connected to said photocell means for shifting the phase of the photocell output signal; `synchronous detector means connected to receive the phase-shifted signal and the photomultiplier output signal as inputs and to provide an A.C. signal output therefrom, the positive portion being proportional to the signals from one of said rst two Windows and the `negative portion being proportional to the signals yfrom the other; narrow-band amplifier means for splitting said A.C. signal into two pants and amplifying each part separately, the iirst part having its positive portion corresponding to the positive portion of Isaid A.C. signal, and the second part having its positive portion corresponding to the negative portion of said A.C. signal, the bandwidth of the `amplifier means allowing passage of the A C. signal trequency but cutting out most of the noise spectrum because of its narrowness; rectiier and iilter mean-s for rectifying each of said ampliied A.C. signals individually to obtain rectified waves consisting of the positive portion thereoffand for obtaining :a steady DuC. signal from each of said rectilied waves; and galvanometer means to which said steady D.C. signals are applied for indicating the difference in amplitude between them;

References Cited in the iile of this patent UNITED'STATES PATENTS 

1. AN INSTRUMENT FOR THE MEASUREMENT OF DENSITY DIFFERENCES IN A MEDIUM EXPOSED TO AN AERODYNAMIC FLOW COMPRISING, IN COMBINATION: SCHLIEREN OPTICAL MEANS FOR PROJECTING A COLLIMATED BEAM OF LIGHT THROUGH SAID MEDIIUM; MEANS FOR DIVIDING SAID LIGHT BEAM INTO TWO PARTS AND FOR INTERRUPTING EACH PART ALTERNATELY; MEANS FOR PRODUCING AN ELECTRICAL OUTPUT SIGNAL PROPORTIONAL TO LIGHT INTENSITY, SAID LIGHT BEAM BEING DIRECTED AT SAID SIGNALPRODUCING MEANS TO PROVIDE AN ELECTRICAL OUTPUT COMPRISING A PULSATING D.C. SIGNAL EVERY SECOND PULSE OF WHICH IS PROPORTIONAL TO THE INTENSITY OF THE SAME PART OF SAID LIGHT BEAM, ALTERNATE PULSES BEING DERIVED FROM DIFFERENT PARTS OF SAID LIGHT BEAM; MEANS FOR OBTAINING AN A.C. SIGNAL HAVING A CHARACTERISTIC PROPORTIONAL TO A CHARACTERISTIC OF SAID PULSATING D.C. SIGNAL; NARROW-BANDWIDTH MEANS FOR PASSING SAID A.C. SIGNAL AND ELIMINATING FREQUENCIES OTHER THAN A NARROW BAND INCLUDING SAID A.C. SIGNAL, THEREBY ELIMINATING THE SPECTRUM OF NOISE SIGNALS OUTSIDE SAID NARROW BAND; MEANS FOR SEPARATING SAID A.C. SIGNAL INTO INDIVIDUAL COMPONENTS, EACH CORRESPONDING TO THE OUTPUT ASSOCIATED WITH A DIFFERENT ONE OF THE PARTS OF SAID LIGHT BEAM, SAID COMPONENTS COMPRISING STEADY D.C. SIGNALS; AND MEANS FOR OBTAINING AND INDICATING THE DIFFERENCE BETWEEN SAID STEADY D.C. SIGNALS. 